Local Atomic Configurations in Intermetallic Crystals: Beyond the First Coordination Shell

We have used a combined geometrical-topological approach to analyze 21,697 intermetallic crystal structures stored in the Inorganic Crystal Structure Database. Following a geometrical scheme of close packing of balls, we have considered the three most typical polyhedral atomic environments of the icosahedral, cuboctahedral, or twinned cuboctahedral shape as well as multi-shell (up to four shells) local atomic configurations (LACs) based on these cores in 10,657 unique crystal structure determinations. In total, half of intermetallic structures have been found to contain one of these configurations, with the icosahedral LACs being the most frequent. We have revealed that even a two-shell configuration strongly predetermines the overall connectivity (topological type) of an intermetallic crystal structure. The chemical and stoichiometric composition of the multi-shell LACs generally obeys the close-packing model: the number of atoms in the subsequent shells (N_k) varies around the value N_k = 10k² + 2, which is valid for the same size atoms, to reach the densest packing for the kth shell. Deviations from the revealed regularities often indicate inconsistencies in the crystallographic information, unusual features of the structure, or the existence of more stable phases that can be used for the validation of experimental and modeling data.

Metal Flux Growth, Structural Relations, and Physical Properties of EuCu2Ge2 and Eu3T2In9 (T = Cu and Ag)

Single crystals (SCs) of the compounds Eu₃Ag₂In₉ and EuCu₂Ge₂ were synthesized through the reactions run in liquid indium. Eu₃Ag₂In₉ crystallizes in the La₃Al₁₁ structure type [orthorhombic space group (SG) Immm] with the lattice parameters: a = 4.8370(1) Å, b = 10.6078(3) Å, and c = 13.9195(4) Å. EuCu₂Ge₂ crystallizes in the tetragonal ThCr₂Si₂ structure type (SG I4/mmm) with the lattice parameters: a = b = 4.2218(1) Å, and c = 10.3394(5) Å. The crystal structure of Eu₃Ag₂In₉ is comprised of edge-shared hexagonal rings consisting of indium. The one-dimensional chains of In₆ rings are shared through the edges, which are further interconnected with other six-membered rings forming a three-dimensional (3D) stable crystal structure along the bc plane. The crystal structure of EuCu₂Ge₂ can be explained as the complex [CuGe]^(2+δ)- polyanionic network embedded with Eu ions. These polyanionic networks present in the crystal structure of EuCu₂Ge₂ are shared through the edges of the 011 plane containing Cu and Ge atoms, resulting in a 3D network. The structural relationship between Eu₃T₂In₉ and EuCu₂Ge₂ has been discussed in detail, and we conclude that Eu₃T₂In₉ is the metal deficient variant of EuCu₂Ge₂. The magnetic susceptibilities of Eu₃T₂In₉ (T = Cu and Ag) and EuCu₂Ge₂ were measured between 2 and 300 K. In all cases, magnetic susceptibility data followed Curie-Weiss law above 150 K. Magnetic moment values obtained from the measurements indicate the probable mixed/intermediate valent behavior of the europium atoms, which was further confirmed by X-ray absorption studies and bond distances around the Eu atoms. Electrical resistivity measurements suggest that Eu₃T₂In₉ and EuCu₂Ge₂ are metallic in nature.

The system Ce–Zn–Si for <33.3 at.% Ce: phase relations, crystal structures and physical properties

The phase relations, mechanism of phase formation, crystal structures, physical properties of novel compounds in the Ce–Zn–Si system were investigated.

Syntheses and Structures of New Phases AeMxIn4-x (Ae = Sr, Ba; M = Mg, Zn): Size Effects and Site Preferences in BaAl4-Type Structures

The title phases were synthesized via high-temperature solid-state methods and structurally characterized by single-crystal X-ray diffraction. The phase widths of both SrMg(x)In(4-x) (0.85 <or= x <or= 1.53) and BaMg(x)In(4-x) (0 <or= x <or= 1.79) are consistent with size matches between Ae and the encapsulating M-In cages. All compounds crystallize in body-centered tetragonal BaAl(4)-type structure (I4/mmm, Z = 2) except that SrMg(x)In(4-x), 0.85 <or= x <or= 1.24, occurs in the lower-symmetry space group (I4m2, Z = 2) and the atoms are all ordered at the composition SrMgIn(3). The substitution of the effectively smaller Mg or Zn atoms into the indium sublattice of the monoclinic SrIn(4) (C2/m) dramatically changes the structure into the tetragonal BaAl(4)-type structure. Compared with the Mg compounds in which all Mg occupy the basal 4d crystallographic site, Zn occupies the other 4e (apical) site in both BaZn(0.79(2))In(3.21) and SrZn(1.00(4))In(3.00). These differences are related to the interplay of size and electronic factors. The latter may be expressed by not only electronegativities but also total energies according to linear muffin-tin-orbital calculations.

Tb4FeGe8 Grown in Liquid Gallium: Trans−Cis Chains from the Distortion of a Planar Ge Square Net

The ternary germanide Tb4FeGe8 was obtained from Ga flux reactions. The crystal structure studied with single-crystal X-ray diffraction revealed the existence of an orthorhombic average substructure (Cmcm, Z=1) with cell parameters a = 4.1118(14) A, b=15.844(5) A, and c=3.9885(15) A. The refinement [I > 2sigma(I)] converged to final residuals R1/wR2 = 0.0363/0.0893. The average structure (CeNiSi2-type) consists of a 3D [Fe1/4Ge2] framework where Ge atoms form a square net and Fe atoms reside alternatively above and below it with only 1/4 occupation probability. X-ray and electron diffraction studies showed a modulation in the Ge net. The modulated structure was refined based on a 4-fold monoclinic supercell (P2(1)/n) with parameters a = 5.7315(11) A, b = 15.842(3) A, c = 11.438(2) A, and beta = 91.724(4) degrees with R1/wR2 = 0.0643/0.1735 and uncovered a severe distortion of the Ge square net. The Ge atoms are displaced to form an array of cis-trans chains. The Ge-Ge distances within these chains are distinctively bonding, whereas those between the chains are nonbonding. Results of the electronic structure calculations and magnetic measurements are also reported. The structural distortions found in Tb4FeGe8 cast a doubt onto the correctness of many of the reported REM1-xGe2 disordered compounds and call for reinvestigation.